1. Field of the Invention
The invention relates generally to switching power supplies and, more particularly, the invention relates to a high efficiency switching controller for use in switching power supplies.
2. Description of Related Technology
Generally speaking, a switching power supply (SPS) provides a cost effective and energy efficient device for converting energy from a single direct current (DC) supply voltage into one or more DC output voltages that have a greater or lesser magnitude than the supply voltage. Traditionally, a SPS has an integrated control circuit that modulates the duty cycle of a transistor switch, which controls the flow of energy into the primary of a transformer to produce one or more desired output voltages that are derived from the secondary of the transformer. As is well known, the energy (i.e., the time integral of power) supplied to the primary of the transformer minus efficiency losses equals the energy transferred to the secondary of the transformer. Thus, if more energy is needed by the secondary, then the control circuit increases the duty cycle of the transistor switch to provide more energy to the primary of the transformer. Conversely, if less energy is needed by the secondary, then the control circuit decreases the duty cycle of the transistor switch.
FIG. 1 is an exemplary schematic block diagram of a conventional SPS, which includes a DC voltage supply block 10, a voltage output block 20, a feedback block 30, and a switching control circuit 40. The DC voltage supply block 10 includes a full wave bridge rectifier 1 and a filter capacitor C1. The bridge rectifier 1 rectifies alternating current (AC) line voltage to produce current pulses which are substantially smoothed to a DC supply voltage by the filter capacitor C1. For example, if the AC line voltage is 110 volts AC, then the smoothed DC supply voltage across capacitor C1 may be approximately 155 volts DC.
The output voltage block 20 includes a switching transformer 22 having a primary winding L1 and secondary windings L2 and L3 and switching rectifier diodes D5 and D6 that receive current pulses from the respective secondary windings L2 and L3 to provide rectified current pulses to respective filter capacitors C2 and C3. The filter capacitors C2 and C3 smooth the rectified current pulses to substantially DC voltages.
The feedback block 30 includes a feedback voltage amplifier 31 and a photo-coupler 32. The feedback voltage amplifier 31 detects the DC voltage across the filter capacitor C2 and provides a proportional current to the photo-coupler 32.
The switching control circuit 40 includes a pulse width modulated (PWM) signal generator 41, a switching transistor M1, a flyback diode D7, and a feedback capacitor C4. The switching transistor M1 is connected to the primary L1 of the transformer 22 and is switched on and off by the PWM signal generator 41 at a duty cycle that is based on the magnitude of a feedback voltage Vfb provided by the feedback capacitor C4.
Initially, when AC line voltage is first provided to the bridge rectifier 1, a supply voltage Vcc applied to the PWM signal generator 41 is substantially near zero volts DC and the PWM signal generator 41 is off. Additionally, because the PWM signal generator 41 is off, the switching transistor M1 is off, energy is not provided to the primary winding L1, and the output voltages across the filter capacitors C2 and C3 are substantially near zero volts DC.
As is generally known, the PWM signal generator 41 is typically fabricated using conventional integrated circuit technologies and requires a relatively low DC supply voltage, which may be, for example, between 4 volts DC and 12 volts DC. Typically, the low supply voltage required by the PWM signal generator 41 is derived from the output voltage block 20. Thus, as shown in FIG. 1, the supply voltage Vcc for the PWM signal generator 41 is provided by the voltage across the filter capacitor C3. Additionally, because the voltage across the filter capacitor C3 is initially substantially near zero volts DC, a start up resistor R is connected between the filter capacitors C1 and C3. The start up resistor R provides an initial charging current to the filter capacitor C3 that causes the voltage across the filter capacitor C3 to increase. When the voltage on the filter capacitor C3 reaches a level sufficient to cause the PWM signal generator 41 to begin functioning, the PWM signal generator 41 regulates the voltage across the filter capacitor C3 and the current flowing through the start up resistor R no longer increases the voltage across the filter capacitor C3.
Although the start up resistor R is needed to the start the operation of the PWM signal generator 41, the start up resistor R becomes a significant source of energy inefficiency once the PWM signal generator 41 is operational. More specifically, a large voltage differential exists across the start up resistor R because the difference between the output voltage of the DC voltage supply block 10 is substantially greater than the low voltage supply Vcc for the PWM signal generator 41. For example, the output voltage of the DC voltage supply block 10 may be 155 volts DC while the low voltage supply Vcc is about 5 volts DC. This large voltage drop across the start up resistor R during continuous operation of the SPS results in a significant source of energy inefficiency.
In accordance with one aspect of the invention, a high efficiency switching controller for use in a switching power supply having a voltage source, a transformer with a primary winding coupled to the voltage source and a secondary winding, a switching transistor coupled to the primary winding, an output voltage circuit coupled to the secondary winding, and a feedback circuit coupled to the output voltage circuit. The high efficiency switching controller includes a current control device coupled to the voltage source and a switch connected between the current control device and the output voltage circuit. The high efficiency switching controller may also include an under voltage lockout regulator coupled to the output voltage circuit and the switch that controls the state of the switch based on a voltage of the output voltage circuit.
The high efficiency switching controller may further include a bias unit coupled to the under voltage lockout regulator that provides current to circuitry within the switching controller based on the voltage of the output voltage circuit, a source/sink unit coupled to the feedback circuit, a first comparator coupled to the source/sink unit, an oscillator coupled to the first comparator, and a pulse width generator coupled to the oscillator and an output of the first comparator that generates a gate drive signal having a duty cycle based on an output of the feedback circuit.
Additionally, the high efficiency switching controller may include a protector coupled to the feedback circuit, the under voltage lockout regulator, and the pulse width generator and an adjuster coupled to the under voltage lockout regulator and the protector. The protector may provide a control signal to the pulse width generator that controls the gate drive signal in response to an operating condition of the switching controller. The operating condition of the switching controller may be associated with a thermal condition or, alternatively, may be associated with an excessive load on the switching controller. Also, the control signal may periodically enable the pulse width generator in response to a voltage of the output voltage circuit so that the gate drive signal includes groups of gate drive pulses.
The high efficiency switching controller may still further include a leading edge blanking unit coupled to the pulse width generator, a second comparator having a first input that monitors a current flowing through the switching transistor and a second input that receives a reference signal, and
an AND gate coupled to the leading edge blanking unit, the second comparator, and the pulse width generator, whereby the AND gate provides a control signal to the pulse width generator that turns the gate drive signal off in response to an overcurrent condition in the switching transistor.
In accordance with another aspect of the invention, an integrated circuit high efficiency switching controller includes a current control device including a transistor that is coupled to a first terminal of the integrated circuit high efficiency switching controller, a switch connected between the current control device and a second terminal of the integrated circuit high efficiency switching controller, and an under voltage lockout regulator connected to the second output terminal and the current control device that controls the conduction of the current control device based on a voltage on the second terminal.